output-management: fix output destroy handling

Currently we update output-management clients based on changes in the
wlr_output_layout struct. However, this is obviously wrong on closer
inspection due to the fact that not all outputs are tracked by the
wlr_output_layout at all times. I think this aproach was originally
cargo-culted from some other output-management implementation and it
needs to go.

Luckily, the solution is quite simple since the only way to configure
outputs using river is through the wlr-output-management protocol. This
means we need to send a new configuration to the output-management
client in 3 cases:

1. An output is created
2. An output is destroyed
3. A wlr-output-management config from a client is applied

river/Output.zig

@@ -409,6 +409,8 @@ fn handleDestroy(listener: *wl.Listener(*wlr.Output), _: *wlr.Output) void {
 
     util.gpa.destroy(output);
 
+    server.root.sendOutputManagerConfig();
+
     server.root.applyPending();
 }